The first thing you do whenever you have a gear problem is find the gear ratio, which is easy enough to do- just divide 45 by 15, which should give you a 3:1 ratio. Since the driver gear has more teeth, it's going to be going slower than the driven gear, so take the 30 and divide it by 3, because the driver gear is going to be going three times as slow, since it's three times as big. Your answer is 10 RPMs.